Vitamin D is produced by the skin of human body when under direct sunlight, and is a ‘fat soluble’ molecule meaning it is stored within the fat tissue in the body.
Vitamin D is a vitamin which helps us in so so many ways, it helps our bones, teeth, immunity, cardiovascular health, diabetes risk, risk of various cancers, pregnancy, children’s growth/ development, and even things like depression, impacting various neurotransmitters which send signals around the brain.
So it’s pretty safe to say that this is a ridiculously important vitamin, and also one which the likelihood of deficiency increases the further away from the equator you live, for the reason that you’re less likely to regularly receive direct sunlight in colder/cloudier environments in particular. Even studies investigating professional footballers playing in the Premier League (the UK!) have found deficiencies at the top level!
There are no fixed guidelines for how much vitamin D, and requirements will likely vary from person to person. During the summer, exposing large patches of skin (face, arms, and legs) for around 10-15 minutes on 2-3 days per week should be sufficient to replenish stores. Although we tend to get most of our vitamin D from the sun, we can also get it from our diet. Oily fish is one of the few foods which are naturally high in vitamin D and it is by far one of the best sources.